Modern aircraft often use a variety of high lift leading and trailing edge devices to improve high angle of attack performance during various phases of flight, for example, takeoff and landing. One such device is a leading edge Krueger flap, shown schematically in FIGS. 1A and 1B. FIG. 1A schematically illustrates a cross-section through a wing 10 having a Krueger flap 22 stowed proximate to the bottom surface of the wing 10. With the Krueger flap 22 in the stowed position, the wing 10 is in a low drag configuration that is suitable for cruise and other low angle of attack operations. FIG. 1B shows the Krueger flap 22 after being rotated and extended forward to create an extension of the leading edge 12 of the wing 10. When the Krueger flap 22 is in this extended position, it improves the air flow characteristics over the wing 10 at high angles of attack, allowing the wing 10 to operate at higher angles of attack without stalling.
When the Krueger flap 22 is in the extended position, it may be separated from the leading edge 12 of the wing 10 by a gap 40. A shortcoming of this arrangement is that while the gap 40 can allow the wing to operate at higher angles of attack, it can also create more drag than a configuration with little or no gap. Accordingly, a Krueger flap arrangement having a gapped extended position may be optimal for the landing phase of flight, but not for the takeoff phase during which a lower drag configuration is desired. Because Krueger flaps typically have only a retracted and an extended position, the extended position is typically optimized for landing performance.
One approach addressing this drawback is to move the Krueger flap to an intermediate position during take-off. When the Krueger flap is in the intermediate position, it is extended, but positioned against the leading edge 12 to seal the gap 40, as indicated by dashed lines in FIG. 1B. Further details of such an arrangement are provided in U.S. Pat. Nos. 5,158,252 and 6,375,126.
One feature of existing three-position Krueger flap arrangements is that they include a single drive tube or actuator coupled to a complex mechanical linkage. A drawback of this feature is that it can increase the weight of the leading edge device and/or reduce the reliability of the leading edge device. A further drawback is that even though the linkage is complex, it may not be capable of positioning the Krueger flap precisely in the most aerodynamically desirable positions. Accordingly, the arrangement may not be aerodynamically efficient, and increasing the efficiency of the arrangement may further increase the complexity and weight of the leading edge device.